Marshall DSL lowa bias 27ma 33ma max

[GP20]

That's the maximum current that i can set for both side.
I pulled 2 tubes.

I had some volume loss, i opened the amp, no C46 cap in this one.
I do the speaker jack mod in order to assure a good contact.
Started again, played it 10 minutes then suddendly loss all the volume.
Shutdown and started again, it worked again without issue this time.
Checked the bias 27 and 33 ma and it's the max i can get with 2 tubes only.
It's a 2001 100w model.


Any idea why the bias is so low?

[nickb]

Any idea why the bias is so low?

Seems to be right on the money according to the data sheet.

[g1]

Marshall calls for 90mV per pair measured at the test points. So with 2 tubes removed you should be able to get 45mV on each side.
Have you checked the screen resistors and screen voltages?

[Mick Bailey]

Are the tubes good to begin with? With good tubes the bias adjustment should give a good range and easily be able to cause the tubes to draw too much current. Low emission tubes can give problems in setting the bias.

[GP20]

Tubes are svetlana EL34 they look good on my tester i will try with a different pair.
If nothing changes with different tubes i will check the plate voltage and screen voltage.

See you soon. ;o)

By the way i keeped the 2 center tubes...i'm right?

[g1]

2 center or 2 outer, shouldn't matter, either way should work. Before you try with the other tubes, move the ones you have now to the 2 outer sockets and recheck. This way we can verify it is not just a problem with the 2 center sockets.
If/when you measure the plate and screen voltage of the power tubes, measure the grids also, so pins3,4, and 5.

[GP20]

Never too late...oups.
Finally i changed the bridge rectifier which seemed to be weak.
It sounds better now though i have to make some checks.